The .delta. receptor has been identified as having a role in many bodily functions such as circulatory and pain systems. Ligands for the .delta. receptor may therefore find potential use as analgesics, and/or as antihypertensive agents. Ligands for the .delta. receptor have also been shown to possess immunomodulatory activities.
The identification of at least three different populations of opioid receptors (.mu., .delta. and .kappa.) is now well established and all three are apparent in both central and peripheral nervous systems of many species including man. Analgesia has been observed in various animal models when one or more of these receptors has been activated.
With few exceptions, currently available selective opioid .delta. ligands are peptidic in nature and are unsuitable for administration by systemic routes. Some non-peptidic .delta. antagonists have been available for some time (see Takemori and Portoghese, 1992, Ann. Rev. Pharmacol. Tox., 32: 239-269. for review). These compounds, e.g. naltrindole, suffer from rather poor (i.e., &lt;10-fold) selectivity for the .delta. receptor vs. .mu. receptor binding and exhibit no analgesic activity, a fact which underscores the need for the development of highly selective non-peptidic .delta. ligands.
Thus, the problem underlying the present invention was to find new analgesics having improved analgesic effects, but also with an improved side-effect profile over current .mu. agonists and potential oral efficacy.
Analgesics that have been identified and are existing in the prior art have many disadvantages in that they suffer from poor pharmacokinetics and are not analgesic when administered by systemic routes. Also, it has been documented that preferred compounds, described within the prior art, show significant convulsive effects when administered systemically.
The problem mentioned above has been solved by developing novel compounds which possess a piperidine ring, which may be a 5-membered, a 6-membered or a 7-membered nitrogen ring, as will be described below.